Micron-sized spherical polymer microparticles are utilized for cosmetic additives, supports for various chemical materials, spacers, column packings for chromatography, light diffusion agents, porosification agents, weight-lightening agents, antiblocking agents, surface modification agents for recording paper, and the like.
Among these, hydrophilic crosslinked polymer microparticles can be used as a hydrous gel microparticle, and is useful as cosmetics additives, supports, porosification agents, weight-lightening agents, and surface modification agents for recording paper.
Production of polymer particles by inverse suspension polymerization of a vinyl-based monomer has conventionally been carried out. As technologies of producing hydrophilic crosslinked polymer particles by inverse suspension polymerization, there have been known a method in which a water-in-oil microdispersed droplet of a monomer is formed using a compound having a specific HLB as a dispersing agent before polymerization and then the monomer is polymerized while dropping it (see Patent Document 1), a method in which inverse suspension polymerization is carried out in the presence of a water-absorptive polymer particle, an oil-soluble polymerization initiator and a dispersing agent, and during or after the polymerization a hydrophobic vinyl-based monomer and an oil-soluble polymerization initiator are added to perform polymerization (see Patent Document 2), a method in which a hydrophilic vinyl-based monomer is inverse suspension polymerized in the presence of a silicone compound having at least one functional group in the reaction system (see Patent Document 3), and the like.
In these conventional technologies, there are problems that the dispersion stability of polymer particles during or after polymerization is not sufficient, the particle size of polymer particles obtained is nonuniform, and the hydrophilicity of polymer particles obtained is degraded. In particular, when hydrophilic particles with a high degree of crosslinking are produced while increasing the proportion of a multifunctional vinyl-based monomer used, polymerization stability is significantly degraded, and problems such as aggregation of particles, degradation in the quality of polymer particles obtained, and a reduction in productivity easily occur.
Since all the above-mentioned production methods are ones in which polymerization is performed by feeding a monomer emulsion continuously over one hour or more to a reactor heated to a high temperature of 70° C. or higher, aggregation of particles or the like easily occurs and the particle size of the resulting polymer particles becomes irregular. In addition, when a large amount of a crosslinking agent such as a multifunctional vinyl-based monomer, is used, most part of unreacted crosslinking agent becomes easy to flow out into a continuous phase side, and when polymerization is continued in this state, particles aggregate more and this is expected to lead to the aforementioned deterioration in quality of polymer particles.
Furthermore, Patent Document 4 discloses an absorptive polymer particle which is produced by inverse suspension polymerization using a redox polymerization initiator for the production of a water absorptive polymer having a specific water absorptivity, and a polymer particle is produced by feeding tert-butyl hydroxyperoxide which is an oil-soluble oxidizing agent, and then feeding sodium bisulfite which is a water-soluble reducing agent.
According to this production method, particle size control of microparticles can be performed more precisely in comparison to aforementioned conventional technologies. Since a polymerization reaction occurs before the water-soluble reducing agent is diffused sufficiently, this is not satisfactory as a method for producing a high-quality particle that is uniform in particle size and has a particle size falling within a specified range, in a stable state without causing, for example, aggregation of particles.    Patent Document 1: JP-A H05-222107    Patent Document 2: JP-A 2003-301019    Patent Document 3: JP-A 2003-34725    Patent Document 4: JP-A 2004-262747